# Calculating daily energy production of PV panels

"Using the example above of a 250-watt STC rated panel, if you multiply the 250 watts the panel produces by the number of hours of full sun you get in a day, you’ll get the amount of kwh that panel produces per day. Multiply by 30 days and you’ll get mothly kWh output for the panel.

The average roof in the United States gets about 4 hours of usable sun per day. We know the sun shines more than 4 hours, but “full sun” is a measurement that combines all the parts of the day when the sun is lower in the sky into one number.

Using 4 hours of full sun, gives you this equation: 250 watts x 4 hours. That’s 1 kWh (1,000 watts) in a day per 250-watt panel.

If you multiply 1kWh per panel by 30 days in a month, you’ll find that each 250 watt rated panel will produce about 30 kWh in an average month."

This makes sense to me, but when I take our soon to be installed 57 305 Watt panels (17.385kW according to my calculations), it doesn't.

With the array size being 17.385kW (69.54kWh generated in an average day, according to the above, if I am not mistaken) to only meet our daily usage of 41.73kwH (1,252kWh is our average monthly usage) out of the array per our solar company, it is only on ever up to 60% efficient.

Does this sound right to others also?

• So, there are never any cloudy or overcast days? – Solar Mike Jun 21 '19 at 18:58
• @SolarMike - Not denying there are cloudy days, but I thought 4 hours on an average day of full sunshine would take this into account, on average... – user66001 Jun 21 '19 at 19:04
• Your figures sound about right. I installed a ground-mounted array in Oct. 2018 consisting of 50 330W (16.5 kW total) panels and a 15kW grid-tied inverter. I live near Indianapolis, IN right at 40 degrees N. My average daily usage on an annual basis is 70 kWh/day. So far the array has produced righ at 51 kWh/day. I expect that by October 2019 it will be really close to the 70 kWh/day design goal. My best day was 116 kWh and my worst was 0.4 kWh. – jwh20 Jun 21 '19 at 19:40
• Pv panels range from 15 to about 22% efficient... not sure what you do to get 60% efficient or do you mean availability? – Solar Mike Jun 21 '19 at 20:53
• Your numbers sound fine, but I don't understand why 41.73 KWH (1252KWH/mo) enters into it. Is that your actual usage, or is that something some solar company is saying? Noting that 41.73 is exactly 60% of your 69.54, so either they trimmed your numbers or did the same calculations with a different fudge factor (2.4h instead of 4h). Regardless, you are asking us to interpret their opinion or divine the reasons for it, and opinion questions are off topic here. Can you revise to a factually answerable question? – Harper - Reinstate Monica Jun 23 '19 at 1:07

"Average hours" on "Average roof" have no particular bearing on a specific roof located in a specific place.

Specific places have specific weather patterns, which do have local averages that can be used for estimation that's a lot more relevant. In the USA, roughly 30 years of month-by month data are typically available for some weather station/airport near you, though exactly how "near you" it is will vary and may impact how applicable the data is. But in general, the Southwest USA is a lot more productive than much of the rest of the country.

The Northeast in November is certainly dire, as I know from several years spent facing the possibility that I'd have to install an off-grid system. That was the "design month" for getting storage and array size to be adequate - eventually circumstances changed and the power company became more amenable to providing a grid connection I could afford.

With a typical grid-tie, you get to take advantage of "excess" production on long sunny summer days, rather than having to make enough power even in November, and then having excess power (and no place to send it) in sunny months.

Specific roofs also have specific relationships to trees, other houses, and terrain; and specific orientations and angles, all of which affect the specific amount of power an array on that specific roof in that specific location will produce. There are tools a solar installer may use to assess those details.

• In other words one can expect anything from 1% output of rated array size (which I trust would never be bought by a consumer interested in putting solar on their roof), or 100% of rated power... for anything from minutes to 4 hours of each day... – user66001 Jun 22 '19 at 21:23
• In summer you may typically get a LOT more than 4 hours useful production, on many days (depending how sunny .vs. overcast your local summer weather is, and the shading or lack therof on your roof) - but longer days are a fact unless you are on the equator and have very little day length variation (but lots of nearly 12-hour days, so...). While in November in my area, you might be lucky to get 5 sunny, short, days. – Ecnerwal Jun 22 '19 at 23:55

Relevant harvest information could be gathered from other PV owners in that specific area. There are many PV forums.

If the focus is on own consumption, and if there is no storage and the feed-in tariff is much lower then the feed-out tariff, an optimal configuration is not to adjust all panels to the south, but to have the panels cover a wide angle from (south-) east to (south-) west according to the daily own power consumption mimic and the tariffs.

That way the power production will better match the demand and sets of panels like the most east and most west ones could be switched in parallel saving inverter costs.